1. Field of the Invention
The present invention relates to a method of reducing catalyst feed tube plugging using preactivated unsupported catalysts. In the method, an unsupported catalyst precursor first is contacted with an activator, or co-catalyst, in a suitable reaction medium, and then the resulting mixture is contacted with additional solvent to form a preactivated unsupported olefin polymerization catalyst composition that can be fed to a gas phase polymerization reactor without plugging the catalyst injection nozzle. Combining the unsupported catalyst precursor, the co-catalyst and any additional solvent in this order prevents tube plugging, and provides a catalyst material that has high activity, avoids forming significant amounts of polymer agglomerates, and avoids reactor fouling.
2. Description of Related Art
Gas phase polymerization of olefin monomers to produce a polyolefins is well known in the art. Various polyolefins can be produced that include homopolymers, copolymers and terploymers of .alpha.-olefins and optionally include dienes, aromatic compounds with vinyl unsaturation and/or carbon monoxide. A catalyst typically is required to initiate polymerization of one or more of the .alpha.-olefin monomers, and the optional dienes, etc. Typical catalysts include, but are not limited to, coordinated anionic catalysts, cationic catalysts, free-radical catalysts, anionic catalysts and the like. As described more fully, inter alia, in U.S. Pat. Nos. 3,779,712, 3,876,602 and 3,023,203, these known catalysts are introduced to the reaction zone as solid particles whereby the active catalyst material is supported on an inert support typically made of alumina, silica and the like. It was generally known in the art that delivering conventional catalysts to a gas phase reactor that were unsupported would result in numerous problems in catalyst delivery, as well as undesirable polymer properties.
Recent developments in the industry, however, have led to the discovery of a class of unsupported catalysts, some of which are typically referred to as metallocenes, or single site catalysts. Delivery of liquid, unsupported catalysts to a gas phase reactor was first described in Brady et al., U.S. Pat. No. 5,317,036, the disclosure of which is incorporated herein by reference in its entirety. Brady recognized disadvantages of supported catalysts including, inter alia, the presence of ash, or residual support material in the polymer which increases the impurity level of the polymer, and a deleterious effect on catalyst activity because not all of the available surface area of the catalyst comes into contact with the reactants. Brady further described a number of advantages attributable to delivering a catalyst to the gas phase reactor in liquid form.
These advantages included a cost savings since there were no costs associated with providing the support material, and processing the support so as to impregnate the active catalyst thereon. In addition, a high catalyst surface area to volume ratio was achieved thereby resulting in improved catalytic activity. Moreover, it was more efficient since the catalytic solid no longer needed to be separated and processed (filtered, washed, dried, etc.), and then handled and transported.
Despite these advantages, the solid catalytic material still needed to be dissolved in a suitable solvent and delivered to the gas phase reactor in the solvent. Many, if not all, of the single site metallocene catalysts which may polymerize olefins, and especially propylene isotactically, such as metallocene dichlorides, are difficult to use because they are insoluble in hydrocarbon solvents such as alkanes. Other unsupported catalysts that may polymerize olefins also are not readily soluble in hydrocarbons, or require significant amounts of hydrocarbon to dissolve the unsupported catalysts. Solvents such as toluene and methylene chloride, although capable of solvating such catalysts, are undesirable because they are toxic in nature and leave undesirable residues. Even in these types of solvents, however, solubilities still can be very low, typically less than 21 mmol/liter in concentration at room temperature. In addition, feeding unsupported catalysts to a gas phase reactor using large quantities of solvents (hydrocarbon or otherwise) often caused reactor fouling to occur, as described, for example, in U.S. Pat. No. 5,240,894.
In addition, when a liquid catalyst is employed in gas phase polymerization, several phenomena can occur. First, the soluble or liquid catalyst tends to deposit on the resin or polymer forming the fluidized bed which in turn leads to accelerated polymerization on the surface of the particles of the bed. As the coated resin particles increase in size, they are exposed to a higher fraction of catalyst solution or spray because of their increased cross-sectional dimensions. If too much catalyst is deposited on the polymer particles, they can grow so large that they cannot be fluidized thereby causing the reactor to be shut down.
Second, using liquid catalyst under conditions of high catalyst activity, e.g., a liquid metallocene catalyst, the initial polymerization rate is often so high that the newly formed polymer or resin particles can soften or melt, adhering to larger particles in the fluidized bed. This needs to be avoided or minimized to avert reactor shutdown.
On the other hand, if the polymer particles size is too small, entrainment can occur resulting in fouling of the recycle line, compressor, and cooler and increased static electricity can occur leading to sheeting, and ultimately, reactor shutdown.
It also was generally thought in the art that introduction of liquid catalyst to a gas phase polymerization would result in small particle sizes, cause undesirable swelling of the polymer or, at the very least, cause aggregation and agglomeration in the particle bed. This agglomeration would undesirably not fluidize well. Agglomerates would plug the product discharge valve, coat the walls of the reactor and form sheets, disrupt the flow of solids and gas in the bed, and generate large chunks that may extend throughout the reactor. Large agglomerates also can form at the point of introduction of the liquid catalyst and plug the catalyst injection nozzle or tube. This may be in part due to the excess amount of hydrocarbon needed to dissolve the unsupported catalysts. Moreover, carry over of excess liquid occurs, causing an undesirable catalyst coating of the walls of the heat exchanger and other downstream equipment with polymer.
It is known to contact single site catalysts that are soluble in hydrocarbons with a coactivating cocatalyst solution prior to administering the catalyst solution to the gas phase reactor, as described, inter alia, U.S. patent application Ser. Nos. 08/781,196 and 08/782,499, the disclosures of which are incorporated by reference herein in their entirety. The amount of hydrocarbon needed to dissolve the catalyst precursor, however, can be high enough to result in an ultimate catalyst solution whose concentration is low enough to result in coating existing resin particles in the gas phase reactor when the catalyst solution is introduced. This coating phenomenon forms undesirable agglomerates and "chunks" of polymer resin material. This problem is exacerbated when the unsupported catalyst is insoluble in hydrocarbons, or only slightly soluble in hydrocarbon solvent.
Preactivating an unsupported catalyst precursor with a co-catalyst may be sufficient to enhance the solubility of the unsupported catalyst, and serves to reduce the need to use toxic solvents, or high quantities of solvent. Plugging of the catalyst feed tube still may occur, however, if the unsupported catalyst and co-catalyst are mixed together after first adding solvent to the co-catalyst prior to mixing with the unsupported catalyst precursor.